This invention relates to a compact apparatus for generating hydrogen. More particularly, this invention relates to a compact hydrogen generating apparatus suitable for use in conjunction with a fuel cell.
Fuel cells convert the chemical energy of a fuel into usable electricity via a chemical reaction and without employing combustion as an intermediate step. Like batteries, fuel cells generate DC current by means of an anode and cathode separated by an ion-transmissive medium. The most common fuel cells are based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. At the anode, hydrogen atoms are split by a catalyst into hydrogen ions (protons) and electrons. The hydrogen ions then travel through the ion-transmissive medium to the cathode. At the same time, the electrons move through an external circuit to a load and then to the cathode. There, the oxygen, hydrogen ions and electrons combine to form water.
One benefit of fuel cells is that the hydrogen they require for operation can be obtained in various ways from renewable sources. Another benefit is that the end products of the fuel cell reaction typically are mostly carbon dioxide and water. Thus, fuel cells have several environmental advantages over internal combustion engines, and therefore have been the subject of much recent research.
Fuel cells operate most efficiently on pure hydrogen. But because hydrogen can be dangerous when stored in quantity and because hydrogen has a low volumetric density compared to fuels such as natural gas, methanol, gasoline or diesel fuel, hydrogen for use in fuel cells for stationary uses generally must be produced at a point near the fuel cell, rather than being produced, stored and distributed from a centralized refining facility. In order for fuel materials other than hydrogen to be utilized by fuel cells, generally a fuel processor must be used to release the hydrogen contained in them. Suitable fuel materials for on-site processing into hydrogen include but are not limited to methanol, ethanol, natural gas, propane, butane, gasoline and diesel fuels. Such fuels are conventionally easy to store and there is a nationwide infrastructure for supplying them.
Particularly when the fuel cell is of the proton exchange membrane (PEM) type, the hydrogen gas is delivered to the fuel cell as a xe2x80x9cwetxe2x80x9d, i.e. water-saturated, gas in order to avoid drying out the membrane. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. Among the earliest PEMs were sulfonated crosswirked polystyrenes. More recently, sulfonated fluorocarbon polymers have been employed. Such PEMs are described in G. E. Wnek et al., New Hydrocarbon Proton Exchange Membranes Based o Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymers, Electrochenical Society Proceedings, Vol. 95-23 (1995), at pages 247 to 251.
Among the methods for producing hydrogen from a fuel material, probably the most common is synthesis gas production, achieved either via steam reforming or partial oxidation. Synthesis gas principally comprises carbon monoxide and hydrogen, but also can contain carbon dioxide and minor amounts of methane and nitrogen. In a conventional steam reforming process, a mixture of desulfurized hydrocarbon feedstock, such as natural gas, and steam are passed at high temperature and elevated pressure over a suitable reforming catalyst, such as a supported nickel catalyst, to facilitate the chemical reaction. When natural gas (methane) is the feedstock, the principal reaction is
CH4+H2O⇄CO+3H2
The concentration of each constituent in the synthesis gas depends on the ratio of steam to hydrocarbon passing over the catalyst, and on the temperature and pressure at which the gases leave the catalyst. The steam reforming reaction is highly endothermic (xcex94H =kJ/mole) that generally requires a large excess of steam and a significant heat source to move the equilibrium to the right. Fuels typically are reformed at a temperature of from about 750xc2x0 to about 950xc2x0 C. (1400xc2x0 to 1800xc2x0 F.) and a pressure of from about 100 kPa to about 7 MPa. Generally, an auxiliary fuel source, which can be either a portion of the feed or the residual fuel exiting the anode, is burned to supply by heat transfer from the hot combustion gases the heat necessary for the steam reforming reaction.
Because current fuel cells require nearly pure hydrogen to function effectively, impurities (primarily carbon monoxide) in the reformer reaction products stream must be removed. Hence, the reformer reaction products themselves are usually further subjected to the reversible xe2x80x9cwater gas shiftxe2x80x9d reaction in which carbon dioxide and hydrogen are produced from carbon monoxide and steam according to the reaction
CO+H2O⇄CO2+H2
Although the water gas shift reaction is somewhat exothermic, the steam reforming process overall remains highly endothermic.
Partial oxidation (POX) reforming also can be used to convert fuel materials into hydrogen; however, this process produces only about 75 percent as much hydrogen compared to steam reforming. The overall partial oxidation reaction for natural gas is
CH4+0.5O2⇄CO+2H2
In a typical partial oxidation reformer, a fuel source and air are combined and ignited and then passed through a partial oxidation catalyst to be converted into carbon dioxide and hydrogen. Controlling the ratio of fuel source to oxygen provides a continuous and mildly exothermic reaction. Partial oxidation reforming typically occurs at a temperature of from about 6500 to about 1300xc2x0 C. and a pressure of from about 1 to about 25 bar. Because the steam reforming reaction is endothermic and occurs only a high temperature, during a cold start of the reforming system, there generally is insufficient hydrogen for the fuel cells until the components of the reformer can be brought up to a sufficient operating temperature. Steam reformers generally have a poor transient response capability. Also, steam reforming processes generally work best on a comparatively large scale, where sophisticated and expensive techniques using volume-intensive equipment can be profitably employed to generate and recover heat. Steam reforming processes thus have not proved to be easily adaptable for use in small-volume, compact systems such as those destined for use in mobile vehicles.
Although partial oxidation reforming processes do not suffer from the drawbacks associated with steam reforming, nevertheless partial oxidation reformers have a different set of problems and thus do not necessarily represent a ready alternative for use in compact systems. For example, fuels produced by partial oxidation reforming contain only about 3545 percent hydrogen, compared to the approximately 70-80 percent hydrogen obtained in fuels produced by steam reforming. Also, the art associated with partial oxidation reformers is not as advanced compared to steam reformers, and it can prove sometimes difficult to find a suitable partial oxidation catalyst for a given feedstock. Thus, many designs based on modifications of steam reforming and partial oxidation processes continue to be proposed.
Systems are known in which certain reforming process components are integrated into a common module. For example, U.S. Pat. No. 5,516,344 discloses a reformer integrated with a shift converter connected downstream of the reformer. A burner associated with the unit combusts a supplied mixture, whereupon the reformer and shift converter are heated by the hot combustion gases.
U.S. Pat. No. 4,925,456 discloses a process and apparatus for producing synthesis gas that employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-reactor and the primary reforming catalyst is positioned either in the central core or in the annulus thereof. The secondary reformer effluent is passed concurrently through whichever of the central core or annulus does not contain the primary reforming catalyst.
U.S. Pat. No. 4,696,871 discloses a method for generating electricity that employs a hydrogen-containing stream produced by a partial oxidation process using compressed air. A hydrocarbon-containing feedstock is combined with steam and air at superatmospheric pressure, with at least one of the reactants preheated by heat transfer from a heated anode waste gas stream.
EP-0654838 discloses a pre-reformer integrated into an apparatus that includes a fuel cell component, whereby during start-up an auxiliary burner is used to heat the reformer and to heat incoming air that is fed to the cell block. The amount of heat added by the auxiliary burner is insufficient to heat the cell to a minimum operating temperature, however, and combustion gas and air are fed into the apparatus and allowed to burn in a chamber adjacent to the cell block.
In a combined reformation and shift reactor disclosed in EP-0600621, the heat generated by a CO shift stage is utilized in a steam generator that is in thermal contact with the shift stage.
WO 96/32188 describes an apparatus comprising two adjacent reaction chambers separated by a heat-conducting partition that provides thermal contact between the chambers. In using the apparatus to convert methane to hydrogen, a methane/air mixture is fed to the first chamber and subjected to a pre-oxidation process using a suitable pre-oxidation catalyst. In the second chamber, a methane/steam mixture is passed through a reforming catalyst. The heat generated in the first reactor is sufficient to supply heat to the endothermic reaction in the second chamber, where heat is passed to the second chamber via the conducting partition.
WO 94/29013 discloses a compact endothermic reaction apparatus in which a plurality of metallic reaction tubes are close-packed inside a reformer vessel. Fuel is burned inside the vessel, which comprises air and fuel distribution means to avoid excessive localized heating of the reaction tubes. Heat is transferred from the flue gas vent and from the reformed gas vent of the reformer to incoming feedstock, fuel, and combustion air. The feedstock is saturated with water and preheated before entering the reaction tubes.
However, for various reasons, these and other previous designs have not proved satisfactory in obtaining an integrated compact reformer that efficiently transfers and recovers heat. The present invention has advantages over those previously disclosed. In particular, the present invention employs the combustion of anode waste gas to improve heat balance and supply heat to a pre-reforming zone, without removing heat from the secondary reforming zone. An oxidation zone is disposed in the flow path between the pre-reforming zone and the secondary reforming zone; and an upper reforming zone is placed between the pre-reforming zone and the oxidation zone to minimize overheating in the oxidation zone by consuming hydrogen in the pre-reformate prior to the oxidation zone. Heat released from the oxidation reaction imparts additional heat to the secondary reforming zone.
One aspect of the present invention relates to compact hydrogen generating apparatus suitable for use in conjunction with a fuel cell comprising a fuel processor reactor having an integrated pre-reforming zone embedded within a secondary reforming zone.
Another aspect of the present invention relates to a compact hydrogen generating apparatus that employs combustion of anode waste gas to supply heat to a pre-reforming zone embodied within a secondary reforming zone, thereby improving the overall heat balance.
Yet another aspect of the present invention relates to a compact hydrogen generating apparatus that employs an oxidation zone disposed in a flow path between a pre-reforming zone and a secondary reforming zone, whereby heat released from the oxidation reaction imparts heat to the secondary reforming zone. A problem was encountered with this combination which was unexpected. It was discovered that hydrogen produced in the pre-reforming zone when combined with an oxygen-containing stream as feed to the oxidation zone resulted in hot spots which sintered the partial oxidation catalyst at the top or inlet of the oxidation zone. The present invention places a low activity reforming catalyst zone between the pre-reforming zone and the oxidation zone which surprisingly prevented the hot spots from forming and extended the life of the partial oxidation catalyst.
Still another aspect of the present invention relates to a compact hydrogen generating apparatus that responds quickly to transient fluctuations in the power demand of a fuel cell.
A still further aspect of the present invention relates to a compact hydrogen generating apparatus that minimizes the overall size of the unit while minimizing the heat loss of the pre-reformer to the environment and from the secondary reforming zone to the pre-reforming zone.
An even further aspect of the present invention relates to a compact hydrogen generating apparatus that converts a fuel to hydrogen in a small volume while minimizing heat losses and providing a safe way to contain a partial oxidation reaction occurring at elevated temperature.
The present invention overcomes the problem of high heat losses from very small process units by disposing a pre-reforming zone and an integrated partial oxidation and secondary reforming zone within a single vessel. In this manner heat losses from high temperature surfaces are minimized, resulting in improved hydrogen purity of the hydrogen product of the fuel processor and improved overall integrated fuel processor and fuel cell efficiency as measured by the net hydrogen efficiency.
One embodiment of the invention is an apparatus for generating hydrogen from a feed stream for use in conjunction with a fuel cell. The apparatus comprises an inner vessel having a longitudinal axis, sides, a first end and a second end opposite. The inner vessel has a mixing zone, an oxidation zone, and a secondary reforming zone. The inner vessel has a layer of insulation disposed surrounding the sides of the inner vessel. A core gas conduit is located outside the inner vessel and radially distributed about the longitudinal axis. The core gas conduit has an interior passage. A catalytic combustion zone is disposed in at least a portion of the interior passage. A plurality of pre-reforming zones comprises an annular pre-reforming catalyst zone which contains a pre-reforming catalyst. Each annular pre-reforming catalyst zone has a terminal end and an annular inlet surface. The annular pre-reforming catalyst zone is disposed annularly surrounding at least a portion of the core gas conduit in thermal communication therewith. The combustion zone is adjacent the terminal end. The core gas conduit extends through the pre-reforming catalyst zone, beyond the terminal end and beyond the annular inlet surface of the annular pre-reforming catalyst zone. A fuel manifold has a fuel inlet in fluid communication therewith. An inner feed plenum is disposed on the first end of the inner vessel in fluid communication with the mixing zone and the terminal end of each annular pre-reforming catalyst zone. The core gas conduit extends through the inner feed plenum to the fuel manifold. The fuel manifold is disposed on the inner feed plenum. The interior passage of the core gas conduit is in fluid communication with the fuel manifold. An outer feed manifold is disposed further distanced from the second end of the inner vessel also partially defining a sealed effluent plenum zone enclosing each pre-reforming catalyst zone and the inner vessel. The sealed effluent plenum zone is in fluid communication with an effluent outlet. The outer feed manifold is in fluid communication with a feed inlet and the annular inlet surface of each pre-reforming catalyst zone. A flue gas manifold is disposed on the feed manifold. The flue gas manifold has a flue gas outlet in fluid communication therewith. The core gas conduit extends through the outer feed manifold to the flue gas manifold. The flue gas manifold is in fluid communication with the interior passage. An air preheating zone is disposed surrounding the effluent plenum zone and the inner feed plenum. The air preheating zone is in fluid communication with an air inlet, the inner feed plenum, and a preheater outlet. The upper reforming zone is disposed on the oxidation zone and the oxidation zone is disposed on the secondary reforming zone. The mixing zone is in fluid communication with the oxidation zone, and the oxidation zone is in fluid communication with the secondary reforming zone.